The structure of (25 − x/2)Li2O · (25 − x/2)Na2O · xPbO · 50P2O5 phosphate glasses (0 ⩽ x ⩽ 50 mol%) has been investigated by Raman spectroscopy, and $^{31}$P-MAS and $^{207}$Pb-VOCS nuclear magnetic resonance. An increase in lead content affects the PO4 tetrahedra network by weakening bonds between phosphorus and non-bridging oxygens. At low concentration, lead behaves as a modifier cation, with large coordination number. For higher PbO contents, a decrease in the coordination number of the Pb2+ cations and an increase in the covalent character of the Pb–O bonds is observed. The changes in density and thermal properties of the glasses are explained through the structural evolution. The non-linear increase of the glass transition and dilatometric softening temperatures with PbO content is related to the non-linear variation of the PbO molar volume. This behavior is discussed in terms of the change in lead coordination when PbO is introduced in a mixed-alkali metaphosphate glass composition.